PROJECT SUMMARY/ABSTRACT Potentially curative treatments for acute myeloid leukemia (AML) are limited to intensive systemic chemotherapy with or without allogeneic bone marrow transplantation (BMT). However, not every patient is healthy enough to tolerate intensive treatments, and not every patient may have a suitably HLA-matched stem cell donor, especially patients from ethnic minority groups. Targeted agents have recently been approved to treat AML, but these usually require intensive systemic chemotherapy to optimize efficacy. Furthermore, AML is genetically heterogeneous with distinct genetic mutations and chromosomal alterations that makes targeted- agent monotherapy unlikely to be curative. AML, like most hematologic malignancies, is very sensitive to radiation therapy but even involved field radiation may be too toxic and ineffective for disseminated systemic disease. However, radioimmunotherapy (RIT) mitigates the off-target toxicity by using monoclonal antibodies conjugated to radioactive isotopes to deliver radiation payloads directly to sites of disease by virtue of the antibody specificity. We have shown that RIT using 90Y- and 131I-radiolabeled anti-CD45 antibody targets radiation to sites of leukemia while minimizing radiation to uninvolved organs. We have improved upon our approach without increasing toxicity by targeting higher energy alpha-emitting radionuclides (astatine-211; 211At) to sites of disease and by developing a pre-targeted RIT (PRIT) approach using bispecific antibodies targeting CD45 and 90Y-DOTA. Using preclinical murine models, we now propose to identify synergistic combinations of 211At- and 90Y-anti- CD45 RIT with novel targeted therapies that interfere with DNA repair or promote apoptosis. We will do this by first assessing for synergy between alpha- or beta-emitting radionuclides (211At- and 90Y-) employed in anti- CD45 directly labeled RIT with recently approved targeted agents (PARP and BCL2 inhibitors) in both disseminated syngeneic and xenograft leukemia murine models. Second, we will improve therapeutic efficacy of anti-CD45 PRIT via bispecific antibody constructs targeting CD45 and 90Y-DOTA by assessing for synergy with targeted therapies (PARP and BCL2 inhibitors) in leukemia murine models. We will characterize the extent of DNA damage achieved with these two approaches as a means to elucidate the mechanism of efficacy. Finally, we will compare these two approaches as part of conditioning prior to allogeneic BMT using haploidentical, or partially matched donors, as all patients should have haploidentical donors. These preclinical studies should readily translate into clinical trials given our infrastructure for NIH funded and pharmaceutical-sponsored clinical trials, using anti-CD45 RIT prior to bone marrow transplantation for aggressive hematologic malignancies. These studies will add effective, well-tolerated treatment options for patients with AML by identifying synergistic combinations of targeted agents with anti-CD45 RIT approaches and by identifying the optimal RIT approach prior to haploidentical BMT.